1. Technical Field of the Invention
This invention relates to a system and method for expanding sound. More particularly, the invention relates to a system that expands sound images from monaural inputs and/or stereo inputs for presentation to left and right output transducers without loss of information content and without altering the tone or stereo balance while maintaining speech centered in the resultant sound image.
2. Description of Related Art
It is now well known that sound images that would normally be directed to two speakers can be manipulated so that the resulting sound output, as perceived by a listener positioned in front of the speakers, will appear to that listener as coming from a location other than from the physical location of the speaker sets, including locations to the left of the left loudspeaker and to the right of the right loudspeaker.
U.S. Pat. Nos. 5,105,462 and 5,208,860, issued to Lowe, et al. on Apr. 14, 1992, and May 4, 1993, respectively, which patents are hereby incorporated by reference herein, are illustrations of systems for positioning monaural sound images at any desired location around a listener. The Lowe patents take a monaural sound image input and position that sound image at a selected location. The systems discussed in the '462 and '860 patents are herein referred to as the Q1 system.
FIG. 4A is an illustration of a filter based upon the disclosure contained in the '462 and '860 patents. The Q1 filter 40 of FIG. 4A will produce a left virtual image. Q1 filter 40 is comprised of an input for a sound signal 410, circuitry for adjusting the phase and amplitude of the input signal 90, and outputs for the altered signal 413 and the unaltered input signal 412.
FIG. 4B illustrates the effect of positioning of a monaural input signal as is accomplished, for example, by the systems disclosed in the '462 and '860 patents. When a monaural input signal is positioned, a listener 453 located in front of a pair of transducers 450, 451 will perceive the sound as coming from a location 452 other than from the physical location of the speakers.
U.S. Pat. No. 5,440,638, issued to Lowe, et al. on Aug. 8, 1995, which patent is hereby incorporated herein by reference, shows a system for enhancing the stereo sound image by removing the common audio information and adding that common information back so that it is preserved in the output sound image heard by the listener. The system described in the '638 patent is referred to as the QX process.
For example, FIG. 5B illustrates the effect of expansion of a stereo input signal. When a stereo input signal is expanded, a listener 553 Apositioned in front of a pair of transducers 550, 551 will perceive an expanded sound field 552 extending beyond the location of the transducers.
Copending U.S. patent application Ser. No. 08/511,788, filed Aug. 7, 1995, discloses a system for enhancing stereo sound images by removing at least a portion of common audio information. An embodiment of the system described in the copending application is shown in FIG. 5A. The systems described in this copending application are also referred to as QX processes.
Prior art sound expansion systems utilize differences between two input signals to expand the input sound. When two input signals are the same, as in a monaural input signal, there is no significant difference between the two input signals for prior art systems to utilize for expansion purposes. Thus, prior art sound expansion systems cannot satisfactorily expand monaural input signals.
Another major problem when attempting to expand sound from a monaural source is that the resultant sound image must sound "real" and not be overly colored.
Thus, an object of this invention is to give a listener the perception that he/she is hearing stereo sound with a full range of highs and lows spread throughout the hearing pattern when the input signals are monaural.
It is further desired to produce a distributed hearing pattern such that the sound image does not appear to come from any one single position.
A further problem in the prior art when attempting to convert monaural sound signals to pseudo stereo is that certain sounds, such as speech, should appear to come from the center as opposed to from one side. Also, in some situations, the character of an input signal will alternate from monaural to true stereo.
Therefore, it is desired to design a system which can accept such an input without changing the position of various instruments when the character of the input is stereo. For example, if the sounds from the horns arrive on the right input, the system should not change the relative position of the horns within the listening field. Further, the system should not split the horn sounds so that part of the sound range comes from the right speaker and part of the sound range comes from the left speaker.
Thus, there is a need in the art for a system and method for accepting a monaural sound image and for creating from that sound image a stereo sound image available for presentation to left and right transducers without coloring the sound output.
A further need exists for such a system that can work on either digital and analog input signals.
A still further need exists in the art for such a system which can accept both monaural as well as stereo inputs without affecting the stereo output image.